The interior walls of buildings, for example houses, offices, restaurants, retail stores, hospitals and the like typically include a frame lined with plasterboard panels. The frame of the wall normally includes a series of upright beams, commonly referred to as studs, to which the plasterboard panels are mounted. The panels are mounted to the studs such that the ends of adjoining panels abut one another. The ends are then covered with wet plaster and subsequently sanded when the plaster dries to provide a continuous wall surface. The wall surface created by the plasterboard panels is also usually painted to provide an aesthetically pleasing appearance.
In general, hard, solid materials, for example plasterboard panels, reflect sound better than softer air permeable materials. In this respect, sound waves incident upon an interior wall lined with plasterboard tend to be reflected well. The reflected sound waves can also undergo reflection by bouncing off other walls and surfaces, even after the source ceases emitting sound. This phenomenon is known as reverberation and the time it takes for reverberant sound energy to dissipate by 60 dB is known as the reverberation time. The reverberation time in an enclosure, for example a room, can make a significant impact upon the intelligibility of speech. In this respect if the reverberation time is too long speech can be difficult to interpret as the reverberant sound in the room acts as background noise.
Ideally, the issue of reverberation is considered and addressed at the design stage of a building. However, in some instances, reverberation problems may not become apparent until construction of a building is completed. In both cases there are various options available to address reverberation issues. These typically include the use of perforated acoustic tiles, carpet, curtains, fabric wall linings and other soft materials. Unfortunately, many of these options are not able to adequately blend with the desired aesthetic appearance.
The acoustic panel disclosed in International Publication No. WO 2009/023900, (herein after referred to as “the Bellmax panel”), the contents of which are herein incorporated by reference, sought to address the issue of aesthetic appearance by providing a sound absorbing acoustic panel which mimicked the look and feel of a conventional plasterboard panel, could be painted like conventional plasterboard yet remained sound absorbing, and be installed using the same installation method as conventional plasterboard. The primary components of the Bellmax panel were a membrane layer made of paper or a polymer film, and an underlining perforated sound absorbing layer preferably made of fibrous polyester material.
Although the Bellmax panel mimicked the look and feel of a conventional plasterboard panel, could be painted like conventional plasterboard, and installed using the same installation method as conventional plasterboard, its ability to absorb sound wave energy once painted was found to be limited to very specific frequencies. In addition, flammability issues made it difficult for the Bellmax panel to adequately comply with stringent building regulations.
FIG. 1 of the accompanying drawings provides a graph showing the absorption coefficient across a range of frequencies for a painted sample Bellmax panel of the prior art. The sample Bellmax panel consisted of a membrane layer made of paper, and a sound absorbing layer made of fibrous polyester having a surface density of approximately 1800 g/m2 (without apertures). The sound absorbing layer had a plurality of 15 mm apertures extending therethrough which provided the layer with 33% open area. The sample was mounted to a frame structure having wall type studs with sound absorbing material having a surface density of approximately 800 g/m2 being located behind the sample in a wall cavity having a depth of 25 mm.
The graph in FIG. 1 demonstrates that the sample Bellmax panel has two prominent absorption peaks at approximately 300 Hz and 1700 Hz with virtually no absorption being provided at other frequencies. The absorption peak at 300 Hz is due to the sample Bellmax panel, in combination with the enclosed air volume in the wall cavity behind the sample, acting as a panel absorber. In this respect, a panel absorber is a form of resonant oscillating mass-spring system whereby the panel is able to resonate in response to sound waves incident on the panel with dampening being provided by the enclosed air volume. The absorption peak at 1700 Hz is due to the portions of the membrane layer which overlie the 15 mm apertures acting as diaphragms which vibrate at maximum amplitude when imparted with sound waves of a frequency corresponding to their resonant frequency, thereby reducing the sound waves energy.
In view of the above, it would be desirable to provide an acoustic panel which is able to absorb sound wave energy across a broad range of frequencies, whilst at the same time mimic the look and feel of a conventional plasterboard panel when painted and be sufficiently nonflammable to comply with building regulations.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art in Australia or any other country on or before the priority date of the claims herein.